guide to uuid generation in java (#1879)

* guide to uuid generation in java

* updated readme.md

core-java/README.md

@@ -102,6 +102,7 @@
 - [How to Perform a Simple HTTP Request in Java](http://www.baeldung.com/java-http-request)
 - [Call Methods at Runtime Using Java Reflection](http://www.baeldung.com/java-method-reflection)
 - [Guide to DelayQueue](http://www.baeldung.com/java-delay-queue)
+- [Guide to UUID in JAVA] (http://www.baeldung.com/guide-to-uuid-in-java)
 - [Comparing getPath(), getAbsolutePath(), and getCanonicalPath() in Java](http://www.baeldung.com/java-path)
 - [How to Add a Single Element to a Stream](http://www.baeldung.com/java-stream-append-prepend)
 - [A Guide to Java SynchronousQueue](http://www.baeldung.com/java-synchronous-queue)
@@ -111,4 +112,3 @@
 - [Difference Between Wait and Sleep in Java](http://www.baeldung.com/java-wait-and-sleep)
 - [LongAdder and LongAccumulator in Java](http://www.baeldung.com/java-longadder-and-longaccumulator)
 - [Using Java MappedByteBuffer](http://www.baeldung.com/java-mapped-byte-buffer)
-

core-java/src/main/java/com/baeldung/uuid/UUIDGenerator.java

@@ -0,0 +1,118 @@
+package com.baeldung.uuid;
+
+import java.io.UnsupportedEncodingException;
+import java.security.MessageDigest;
+import java.security.NoSuchAlgorithmException;
+import java.util.UUID;
+
+public class UUIDGenerator {
+    
+    /**
+     * These are predefined UUID for name spaces
+     */
+    private static final String NAMESPACE_DNS   = "6ba7b810-9dad-11d1-80b4-00c04fd430c8";
+    private static final String NAMESPACE_URL   = "6ba7b811-9dad-11d1-80b4-00c04fd430c8";
+    private static final String NAMESPACE_OID   = "6ba7b812-9dad-11d1-80b4-00c04fd430c8";
+    private static final String NAMESPACE_X500  = "6ba7b814-9dad-11d1-80b4-00c04fd430c8";
+
+    private static final  char[] hexArray = "0123456789ABCDEF".toCharArray();
+
+    public static void main(String[] args) {
+        try {
+            System.out.println("Type 3 : " + generateType3UUID(NAMESPACE_DNS, "google.com"));
+            System.out.println("Type 4 : " + generateType4UUID());
+            System.out.println("Type 5 : " + generateType5UUID(NAMESPACE_URL, "google.com"));
+            System.out.println("Unique key  : " + generateUniqueKeysWithUUIDAndMessageDigest());
+        } catch (NoSuchAlgorithmException | UnsupportedEncodingException e) {
+            e.printStackTrace();
+        }
+    }
+
+    /**
+     * Type 4 UUID Generation
+     */
+    public static UUID generateType4UUID() {
+        UUID uuid = UUID.randomUUID();
+        return uuid;
+    }
+    
+    /**
+     * Type 3 UUID Generation
+     * 
+     * @throws UnsupportedEncodingException 
+     */
+    public static UUID generateType3UUID(String namespace, String name) throws UnsupportedEncodingException {
+        String source = namespace + name;
+        byte[] bytes = source.getBytes("UTF-8");
+        UUID uuid = UUID.nameUUIDFromBytes(bytes);
+        return uuid;
+    }
+    
+    /**
+     * Type 5 UUID Generation
+     * 
+     * @throws UnsupportedEncodingException 
+     */
+    public static UUID generateType5UUID(String namespace, String name) throws UnsupportedEncodingException {
+        String source = namespace + name;
+        byte[] bytes = source.getBytes("UTF-8");
+        UUID uuid = type5UUIDFromBytes(bytes);
+        return uuid;
+    }
+    
+    
+    public static UUID type5UUIDFromBytes(byte[] name) {
+        MessageDigest md;
+        try {
+            md = MessageDigest.getInstance("SHA-1");
+        } catch (NoSuchAlgorithmException nsae) {
+            throw new InternalError("MD5 not supported", nsae);
+        }
+        byte[] bytes = md.digest(name);
+        bytes[6]  &= 0x0f;  /* clear version        */
+        bytes[6]  |= 0x50;  /* set to version 5     */
+        bytes[8]  &= 0x3f;  /* clear variant        */
+        bytes[8]  |= 0x80;  /* set to IETF variant  */
+        return constructType5UUID(bytes);
+    }
+    
+    private static UUID constructType5UUID(byte[] data) {
+        long msb = 0;
+        long lsb = 0;
+        assert data.length == 16 : "data must be 16 bytes in length";
+        
+        for (int i=0; i<8; i++)
+            msb = (msb << 8) | (data[i] & 0xff);
+        
+        for (int i=8; i<16; i++)
+            lsb = (lsb << 8) | (data[i] & 0xff);
+        return new UUID(msb, lsb);
+    }
+
+    
+    /**
+     * Unique Keys Generation Using Message Digest and Type 4 UUID
+     * 
+     * @throws NoSuchAlgorithmException 
+     * @throws UnsupportedEncodingException 
+     */
+    public static String generateUniqueKeysWithUUIDAndMessageDigest() throws NoSuchAlgorithmException, UnsupportedEncodingException {
+        MessageDigest salt = MessageDigest.getInstance("SHA-256");
+        salt.update(UUID.randomUUID()
+            .toString()
+            .getBytes("UTF-8"));
+        String digest = bytesToHex(salt.digest());
+        return digest;
+    }
+
+    public static String bytesToHex(byte[] bytes) {
+        char[] hexChars = new char[bytes.length * 2];
+        for (int j = 0; j < bytes.length; j++) {
+            int v = bytes[j] & 0xFF;
+            hexChars[j * 2] = hexArray[v >>> 4];
+            hexChars[j * 2 + 1] = hexArray[v & 0x0F];
+        }
+        return new String(hexChars);
+    }
+
+}